Galvanically isolated dc/dc converter and method of controlling a galvanically isolated dc/dc converter

ABSTRACT

A galvanically isolated DC/DC converter includes a first and a second side converter circuit coupled between a pair of first side DC terminals and a pair of second side DC terminals, respectively. The first side converter circuit has a first and a second switching element, each including a switch and a diode. When the DC/DC converter is in power transfer operation from the second side DC terminals to the first side DC terminals, the second side converter circuit alternates between two power transfer states. A conductive state of the diode of one of the first and second switching elements is the result of one of the two power transfer states. The first side converter circuit is controlled such that the switch of the respectively other of the first and second switching elements is closed for an adaptation interval before the one of the two power transfer states starts.

RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §371 ofInternational Patent Application No. PCT/EP/2011/060614, having aninternational filing date of Jun. 24, 2011, the content of which isincorporated herein by reference in its entirety.

FIELD

The present application relates to DC/DC converters.

BACKGROUND

DC/DC converters are in widespread use today. A typical applicationexample is the coupling of a DC power source to a rechargeable batteryfor charging the same via the DC/DC converter with a desired voltage.

A previous approach DC/DC converter is shown in FIG. 1. Therein, a pairof first side terminals 10 is coupled to a first side converter circuit20, which in turn is coupled to a transformer circuit 30, which in turnis coupled to a second side converter circuit 240, which in turn iscoupled to a pair of second side DC terminals 60. Via the transformercircuit 30, galvanic isolation between the pair of first side DCterminals 10 and the pair of second side terminals 60 is achieved.

Both of the first side converter circuit 20 and the second sideconverter circuit 240 comprise an H bridge circuit. Each H bridgecircuit comprises four switching elements. Each switching element isdepicted to have a switch and a reverse directed diode in parallel.

In a case where power is transferred from the pair of second side DCterminals 60 to the pair of first side DC terminals 10, power istransferred in two different power transfer states. As the power flow isfrom the pair of second side DC terminals 60 to the pair of first sideDC terminals 10, this power flow direction is also referred to as areverse power transfer, with the two power transfer states being alsoreferred to as reverse power transfer states. In a first power transferstate, switches 242 and 248 are closed, and a current flow through thetwo diodes 23 and 29 in the first side converter circuit 20 isestablished. In a second power transfer state, switches 244 and 246 areclosed, and a current flow through the two diodes 24 and 26 in the firstside converter circuit 20 is established. The DC/DC converter 2 iscontrolled to alternate between these two power transfer states,transferring power through the transformer circuit 30 in a galvanicallyisolated manner.

In previous approach DC/DC converters, the response time of the firstside converter circuit with respect to the second side converter circuitentering one of the first and second power transfer states has not beensatisfying. To the contrary, there have been issues with undesiredvoltage peaks in the first side converter circuit as a response to aswitching of power transfer states in the second side converter circuit.

SUMMARY

Accordingly, it would be beneficial to provide a galvanically isolatedDC/DC converter and a method of controlling a galvanically isolatedDC/DC converter that improve the power absorption properties of thefirst side converter circuit during a power transfer from the pair ofsecond side DC terminals to the pair of first side DC terminals.

This problem is solved by the galvanically isolated DC/DC converter inaccordance with claim 1.

The claimed galvanically isolated DC/DC converter comprises at least onefirst side converter circuit coupled between a pair of first side DCterminals, the first side converter circuit having at least a first anda second switching element, with each of the first and second switchingelements comprising a switch and a diode connected in parallel, and atleast one second side converter circuit coupled between a pair of secondside DC terminals, wherein, when the DC/DC converter is in powertransfer operation from the pair of second side DC terminals to the pairof first side DC terminals, the second side converter circuit is adaptedto alternate between two power transfer states, wherein a conductivestate of the diode of one of the first and second switching elements isthe result of one of the two power transfer states, with the first sideconverter circuit being controlled such that the switch of therespectively other of the first and second switching elements is closedfor an adaptation interval prior to a beginning of the one of the twopower transfer states.

The wording to alternate between two power transfer states is not to beunderstood in a way that at any given point in time one of the first andsecond power transfer states has to be present. There are generallyintervals of no power transfer between the intervals of the first andsecond power transfer states. The galvanic isolation may be implementedby a transformer being coupled between the first side converter circuitand the second side converter circuit.

The timing relationship between the power transfer states and theclosing times of the switches can alternatively be described as follows.The conductive state of the diode of the first switching element of thefirst side converter circuit is the result of the first power transferstate. The first side converter circuit is controlled such that theswitch of the second switching element is closed for the adaptationinterval prior to the beginning of the first power transfer state. Theconductive state of the diode of the second switching element of thefirst side converter circuit is the result of the second power transferstate. The first side converter circuit is controlled such that theswitch of the first switching element is closed for the adaptationinterval prior to the beginning of the second power transfer state.

The conductive state of the diode of the first switching element is adirect consequence of the first power transfer state being present.Accordingly, the duration of the conductive state of the diode of thefirst switching element is substantially equal to the interval of thefirst power transfer state. Analogously, the conductive state of thediode of the second switching element is a direct consequence of thesecond power transfer state being present. Accordingly, the duration ofthe conductive state of the diode of the second switching element issubstantially equal to the interval of the second power transfer state.

The first and second switching elements may be coupled in series betweenthe pair of first side DC terminals. The connection point between thefirst and second switching elements may be coupled to one end of thefirst side transformer winding of a transformer coupling the first sideconverter circuit and the second side converter circuit.

The current established during the adaptation interval allows for asmooth onset of the current flow through the first side convertercircuit at the beginning of a respective power transfer state. Inparticular, the closing of the switch of the first switching elementgives rise to a current flow through the first side converter circuitfrom the high potential terminal of the first side DC terminals to thelow potential terminal of the first side DC terminals. After the switchof the first switching element is opened, this current flow commutatesin a way to flow through the diode of the second switching elementtowards the high potential terminal of the first side DC terminals. Inthis way, a current flow through the diode of the second switchingelement is already present when the second power transfer state isentered. Accordingly, the second switching element is pre-conditioned,such that the current flow, which is to be induced by the second powertransfer state, has already been started before the onset of the secondpower transfer state.

In particular, the parasitic capacitances of the diode of the secondswitching element, especially the junction capacitance of the diode ofthe second switching element, are partially or completely charged duringthe adaptation interval. The first side converter circuit does thereforenot exhibit an undesirably high impedance at the onset of the firstpower transfer state. Also, the first side converter circuit alonecomprises less parasitic connection inductances than the first sideconverter circuit and the second side converter circuit together, suchthat the transfer of the diode of the second switching element into aconductive state can be done by above described pre-conditioning with noor less over-voltages as compared to when caused by the second sideconverter circuit. The current flow through the diode of the secondswitching element starts smoothly, and no over-voltages are induced atthe parasitic inductances of the first side converter circuit.

The same smooth current flow behavior is achieved for the first powertransfer state in an analogous manner.

The DC/DC converter may comprise a control circuit, which is configuredto control the at least one first side converter circuit and the atleast one second side converter circuit. In particular, the controlcircuit may be configured to generate a first side converter controlsignal, which controls each of the first and second switching elementsof the first side converter circuit such that a closed state of therespective switch is present during the adaptation interval. The controlcircuit may further be configured to generate a second side convertercontrol signal controlling the at least one second side convertercircuit to be in the first power transfer state or in the second powertransfer state or in a state of no power transfer.

According to a further embodiment, the first side converter circuit isan H bridge circuit, which comprises the first and second switchingelements and further comprises a third and a fourth switching element,with each of the third and fourth switching elements comprising a switchand a diode in parallel, wherein a conductive state of the diodes of twoof the first to fourth switching elements is the result of one of thetwo power transfer states, with the first side converter circuit beingcontrolled such that the switches of the respectively other two of thefirst to fourth switching elements are closed for the adaptationinterval prior to the beginning of the one of the two power transferstates.

In other words: the conductive state of the diodes of the first andfourth switching elements is the result of the first power transferstate. The first side converter circuit is controlled such that theswitches of the second and third switching elements are closed for theadaptation interval prior to the beginning of the first power transferstate. The conductive states of the diodes of the second and thirdswitching elements are the result of the second power transfer state.The first side converter circuit is controlled such that the switches ofthe first and fourth switching elements are closed for the adaptationinterval prior to the beginning of the second power transfer state.

Accordingly, the fourth switching element is controlled to behave likethe first switching element, and the third switching element iscontrolled to behave like the second switching element.

The diodes of the respective two of the first to fourth switchingelements which are conductive at the same time form a diagonal of the Hbridge circuit. In this way, the current flow through the first sidetransformer winding of the transformer and through the first sideconverter circuit transfers power to the pair of first side DCterminals.

Accordingly, the same control signals are applied to the first andfourth switching elements as well as to the second and third switchingelements. The H bridge is comprised of four identical switchingelements. It is, however, also possible that the third and fourthswitching elements are replaced with capacitors. In this way, the firstand second switching elements are the only switching elements in thefirst side converter circuit.

According to a further embodiment, the second side converter circuit isadapted to be in a state of no power transfer between the two powertransfer states, with the adaptation interval being during the state ofno power transfer. In this way, the first side converter circuit can bepre-conditioned for the upcoming power transfer during a time framewhere no current flow is induced from the second side converter circuit.Accordingly, the pre-conditioning can be controlled accurately, whileonly a small amount of energy has to be invested for pre-conditioningthe first side converter circuit.

According to a further embodiment. the first side converter circuit iscontrolled such that the adaptation interval ends a preset commutationtime before the beginning of the one of the two power transfer states.In this way, it is ensured that the current flow through the first sideconverter circuit commutes without disturbance from the switch(es) ofthe first and, if present, fourth switching elements to the diode(s) ofthe second and, if present, third switching elements as well as from theswitch(es) of the second and, if present, third switching elements tothe diode(s) of the first and, if present, fourth switching elements,before the respective power transfer state is entered.

According to a further embodiment, the preset commutation time is chosensuch that the conductive state of the diode of the one of the first andsecond switching elements is present at the beginning of the one of thetwo power transfer states. In other words, the preset commutation timeis at least long enough that it accounts for the switching delay of thediodes. In particular, it is long enough that the parasitic capacitancesof the respective diode(s) are charged and the current flow through therespective diode(s), induced from the second side converter circuit,starts at the latest with the beginning of the respective power transferstate.

According to a further embodiment, the adaptation interval has a presetduration. In this way, the control efforts for controlling the firstside converter circuit are kept low. The duration of the adaptationinterval is the same for all operating conditions. The preset durationof the adaptation interval may be chosen in a way that voltage peakswithin the first side converter circuit are reduced in an optimizedmanner for the operating point of the DC/DC converter that would yieldthe highest voltage peaks within the first side converter circuitwithout closing the respective switches during the adaptation interval.In this way, it is ensured that the undesired voltage peaks are kept atan acceptable limit even for the operating conditions mostdisadvantageous for the first side converter circuit.

According to a further embodiment, a duration of the adaptation intervalis dependent on the operating point of the DC/DC converter. In this way,a current flow of a desired magnitude can be generated during theadaptation interval. This current flow, after commutating to therespective diode(s), will then have a desired magnitude at the onset ofthe respective power transfer state. Depending on the voltage at thepair of second side DC terminals, the voltage at the pair of first sideDC terminals, the particular circuit configuration and other factors,the induced current at the onset of a power transfer state may vary. Bymaking the adaptation interval dependent on the current operatingconditions, the optimized power transfer onset behavior may beimplemented well over a wide range of operating scenarios.

According to a further embodiment, each of the switches of the firstside converter circuit is an insulated-gate bipolar transistor. Withinsulated-gate bipolar transistors, high voltages, such as between 400 Vto 800 V across the first side DC terminals may be supported. For highvoltages, accordingly dimensioned insulated-gate bipolar transistors arecheaper than accordingly dimensioned MOSFETs, such that the DC/DCconverter can be manufactured at a much more affordable price.

According to a further embodiment, the first side converter circuit hasvoltage source characteristics. The first side converter circuit maycomprise a first side capacitor for providing the voltage sourcecharacteristics. The voltage source behavior allows for a current flowfrom the high potential terminal to the low potential terminal of thefirst side DC terminals during the adaptation interval that becomes acurrent flow from the low potential terminal to the high potentialterminal of the first side DC terminals after the adaptation interval.

According to a further embodiment, the second side converter circuit hascurrent source characteristics. The current source characteristic maystem from one or more circuit elements, such as one or more inductanceelements present in each second side converter circuit. With the secondside converter circuit having current source characteristic, the designmay be matched to the first side converter circuit and the transformercircuit having voltage source characteristics. The DC/DC converter as awhole may thus comprise at least one element with current sourcecharacteristic in combination with element(s) causing voltage sourcecharacteristic, allowing a stable continuous operation.

According to a further embodiment, the second side converter circuit hastwo parallel branches between the pair of second side DC terminals, eachbranch comprising an inductance element and a second side switchingelement. In this case, the transformer, which couples the first sideconverter circuit and the second side converter circuit, may be coupledbetween the two connection points of the two parallel branches, i.e.between the respective connection points of the second side switchingelements and the inductance elements. In particular, the conductiondirection of the diodes of the second side switching elements may befrom the low potential terminal of the second side DC terminals towardsthe connection point of the respective branch. In each of the two powertransfer modes, the current flow through the second side convertercircuit goes through a second side switching element in one of the twobranches, the transformer and the inductance element of the respectivelyother branch. The arrangement of the second side converter circuitallows for a low component realization thereof, requiring only twosecond side switching elements and two inductance elements. Also, thistopology allows for the control signals of the second side switchingelements being related to a common ground and for a simple transformertopology.

According to a further embodiment, the second side converter circuit isan H bridge circuit or a transformer center tapped circuit having atleast two second side switching elements. These circuits may be coupledas second side converter circuits alternatively to the circuit havingtwo parallel branches described above. A transformer center tappedcircuit comprises an inductance element between a center point of thesecond side transformer winding and the low potential terminal of thesecond side DC terminals, while the two ends of the second sidetransformer winding are coupled to the high potential terminal of thesecond side DC terminals via a respective second side switching element.

In all of the topologies of the second side converter circuit describedabove, the orientation of the second side switching elements may bereversed, leading to a switching of the high and low potential terminalsof the second side DC terminals.

According to a further embodiment, each of the second side switchingelements is a MOSFET. MOSFETs can be used well in application scenarioshaving a comparably low voltage across the pair of second side DCterminals without generating an unacceptable voltage drop in relation tothe voltage across the pair of second side DC terminals. Using a MOSFET,the internal parasitic body diode thereof may be used or an additionaldiscrete diode may be connected in parallel to the MOSFET. In the lattercase, an additional Schottky diode may be connected in series with theMOSFET to help in blocking the action of the internal parasitic bodydiode of the MOSFET.

According to a further embodiment, the second side converter circuit hasat least two second side switching elements, with each of a first oneand a second one of the second side switching elements comprising aswitch and a diode connected in parallel, wherein, when the DC/DCconverter is in forward power transfer operation from the pair of firstside DC terminals to the pair of second side DC terminals, the diodes ofthe first one and the second one of the second side switching elementsare alternately in a conductive state, with each of the first one andthe second one of the second side switching elements being controlledsuch that a closed state of the respective switch extends beyond atransitioning of the diode of the same second side switching elementfrom the conductive state to a blocking state.

In the forward power transfer operation from the pair of first side DCterminals to the pair of second side DC terminals, the first sideconverter circuit is controlled to alternate between a first and asecond forward power transfer state. This is not to be understood in away that at any given point in time one of the first and second forwardpower transfer states has to be present. There may be intervals of nopower transfer between the intervals of the first and second forwardpower transfer states.

In the first forward power transfer state, the diode of the first one ofthe second side switching elements is in a conductive state. Theconductive state is a direct consequence of the first forward powertransfer state being present. Accordingly, the duration of theconductive state of the diode of the first one of the second sideswitching elements is substantially equal to the interval of the firstforward power transfer state. During the first forward power transferstate, a charge is built up at the diode of the first one of the secondside switching elements.

For at least a portion of the time the diode of the first one of thesecond side switching elements is in the conductive state, the switch ofthe first one of the second side switching elements is controlled to bein a conductive state. When the first forward power transfer state iscontrolled to end, the diode of the first one of the second sideswitching elements leaves the conductive state. As stated above, theswitch of the first one of the second side switching elements iscontrolled to remain in the conductive state for the transitioning ofthe diode of the first one of the second side switching elements fromthe conductive state to the blocking state.

After the end of the first forward power transfer state, thetransitioning from the conductive state to the blocking state of thediode of the first one of the second side switching elements is causedby the voltage across the pair of second side DC terminals, which applya reverse voltage to said diode. Parasitic charges, especially chargesat the junction capacitance of the diode of the first one of the secondside switching elements, are removed, giving rise to a reverse currentas compared to the current through said diode during the first forwardpower transfer state. This reverse current, which is also referred to asdischarge current, would stop abruptly, which is also referred to asdiode cutoff, and the parasitic inductances, which work to maintain thisreversed current, would lead to voltage peaks without keeping the switchof the first one of the second side switching elements closed. Keepingsaid switch closed results in easing the impact of diode cutoff throughproviding a parallel conductive path. Therefore, the voltage peaks ofprevious approach implementations that were dangerous to thesemiconductor elements of the second side converter circuit may beprevented or kept at an acceptable limit.

The extension of the closed state of the switch within the respectivesecond side switching element eliminates the need for further circuitcomponents for preventing the voltage peaks. While previous approachesarranged further diodes in series and in parallel with the second sideswitching elements in order to avoid the voltage peak problem, thedescribed behavior compensates for the detrimental dynamic properties ofthe diodes of the second side switching elements by the control of therespectively associated switches. In this way, the number of circuitcomponents is kept low, which keeps the cost of the DC/DC converter andelectric losses during the operation of the DC/DC converter low.

The operation in the second forward power transfer state is analogous.When the first side converter circuit is controlled to be in the secondforward power transfer state, the diode of the second one of the secondside switching elements is in the conductive state. For at least aportion of the duration of the conductive state of the diode of thesecond one of the second side switching elements, the switch of thesecond one of the second side switching elements is in a conductivestate as well. When the second forward power transfer state iscontrolled to end and the diode of the second one of the second sideswitching elements transfers from the conductive state to a blockingstate, the switch of the second one of the second side switchingelements is controlled to remain in the closed state beyond theconductive state of the diode of the second one of the second sideswitching elements.

The DC/DC converter may comprise a control circuit, which is configuredto control the at least one first side converter circuit and the atleast one second side converter circuit. In particular, the controlcircuit may be configured to generate a first side converter controlsignal controlling the at least one first side converter circuit to bein the first forward power transfer state or in the second forward powertransfer state or in a state of no power transfer. The control circuitmay further be configured to generate a second side converter controlsignal, which controls each of the first one and the second one of thesecond side switching elements of the second side converter circuit suchthat a closed state of the respective switch extends beyond atransitioning of the diode of the same second side switching elementfrom the conductive state to a blocking state.

According to a further embodiment, the respective switch of each of thefirst one and the second one of the second side switching elements iscontrolled to condition a slope of a discharge current of the diode ofthe same second side switching element during the transitioning thereoffrom the conductive state to the blocking state. In particular, theslope of the discharge current may be conditioned to not have an abruptending. The respective switch of each of the first one and the secondone of the second side switching elements may have an adjustableresistance. A charge is built up at the diode during the conductivestate. The built-up charge leads to the discharge current, which is alsoreferred to as reverse current, when the diode is transferred into theblocking state. In previous approaches, when the switch was closed priorto the diode entering the blocking state, an uncontrolled ending of thisreverse current led to unacceptable voltage peaks. By controlling thecurrent slope of the reverse current by the respective switch, thereverse current is confined, such that the voltage peak can at least bekept at an acceptable limit. By conditioning the discharge currentslope, the negative effects of the diode cutoff may be dealt with viathe control of the switch of the respective second side switchingelement in an optimized manner.

The control of the second side switching elements in such a way that themagnitude of the reverse current of the respective diode is conditionedcan be reached in various ways. This may for example be done bycontrolling the channels of the MOSFETs to have a particular resistance.The respective switch may still be closed, but exhibit the particularresistance along its channel. In this way, the discharge process mayalso be adapted to different usage or application scenarios, such asdifferent times of no power transfer between the two forward powertransfer states.

According to a further embodiment, each of the first one and the secondone of the second side switching elements is controlled such that therespective switch is in the conductive state during substantially thewhole time the diode of the same second side switching element is in theconductive state. In this way, the benefit of reducing the resistance ofthe second side switching elements through the provision of two parallelconductive elements, namely the diode and the switch, is used duringsubstantially the whole time of the respective forward power transferstate. The term substantially the whole time is understood to includethe case that the switch is brought to the conductive state a delay timeafter the diode enters the conductive state. Such a delay time may beprovided to prevent short circuit connections within the second sideconverter circuit. The delay time may be much shorter than the durationof the conductive state of the diode, in particular it may be less than10%, more particularly less than 5% of the duration of the conductivestate of the diode.

According to a further embodiment, the first side converter circuit isadapted to alternate between two forward power transfer states, whereineach forward power transfer state leads to the conductive state of oneof the diodes of the first one and the second one of the second sideswitching elements, with each of the first one and the second one of thesecond side switching elements being controlled such that the respectiveswitch is opened a preset lag time after the respective one of the twoforward power transfer states is entered that leads to the conductivestate of the diode of the other second side switching element. In otherwords, the first side converter circuit is adapted to impose analternating current flow in the second side converter circuit, with thesecond side converter circuit converting the alternating current flow tothe DC voltage present at the pair of second side DC terminals. A firstdirection of the alternating current flow is associated with theconductive state of the diode of the first one of the second sideswitching elements and a second direction of the alternating currentflow is associated with the conductive state of the diode of the secondone of the second side switching elements. Accordingly, the firstforward power transfer state results in the diode of the first one ofthe second side switching elements being conductive. The switch of thefirst one of the second side switching elements is closed, such that thediode and the switch both carry a portion of the current flow. When thefirst forward power transfer state is ended, a state of no powertransfer is generally entered. The diode of the first one of the secondside switching elements transitions from the conductive state to theblocking state. The switch of the first one of the second side switchingelements is kept closed. From the state of no power transfer, the firstside converter circuit enters the second forward power transfer state,as a result of which the diode of the second one of the second sideswitching elements enters a conductive state. The switch of the firstone of the second side switching elements is opened a preset lag timeafter the second forward power transfer state is entered. As laid outabove, the terminology of alternating between two forward power transferstates does not imply that one of these two states is present at anygiven point in time during the operation of the DC/DC converter. Ingeneral, a state of no power transfer is present in between eachtransition between the two forward power transfer states. However, theduration of the state of no power transfer depends on the particularembodiment of the DC/DC converter and the power to be transferred in thecurrent operation conditions. This duration is therefore variable andmay be set to zero in particular embodiments or operating conditions.With the respective switch being opened a preset lag time after theforward power transfer state is entered which leads to the conductivestate of the diode of the other of the second side switching elements,it is ensured that at least the preset lag time is dedicated toachieving the discharge at the diode in a desired manner, for example byachieving a controlled reverse current slope at the diode, such that areduction to acceptable voltage peaks can be achieved under alloperating conditions even when the first side converter circuit directlyswitches between the two forward power transfer states without enteringthe state of no power transfer therebetween.

According to a further embodiment, the first side converter circuit isadapted to alternate between two forward power transfer states, whereineach forward power transfer state leads to the conductive state of oneof the diodes of the first one and the second one of the second sideswitching elements, with each of the first one and the second one of thesecond side switching elements being controlled such that the respectiveswitch is closed a preset delay time after the respective one of the twoforward power transfer states is entered that leads to the conductivestate of the diode of the same second side switching element. In thisway, it is ensured that the current flow through the diode isestablished in the desired direction before the resistance of therespective second side switching element is reduced by closing therespective switch.

According to a further embodiment, the preset delay time is greater thanthe preset lag time. In this way, it is ensured that only one of theswitches of the first one and the second one of the second sideswitching elements is closed at any given point in time. Accordingly, noshort circuit or low resistance connection between the pair of secondside terminals is present at any point in time during the forward powertransfer. Safe operation is achieved, while it is made possible that therespective diode is discharged during the preset lag time in the casewhen the first side converter circuit switches directly between the twoforward power transfer states or in the case when the first sideconverter circuit switches between the state of no power transfer andone of the two forward power transfer states.

According to a further embodiment, the second side converter circuit iscoupled to a protection circuit. Such a protection circuit may beprovided in order to protect the second side converter circuit againstvoltage peaks and over-voltage, respectively. It may be provided as analternative to or as a supplement to the extension of the closed stateof the switch of the first one and the second one of the second sideswitching elements beyond the transitioning of the associated diode fromthe conductive state to the blocking state. In particular, the durationof the closed state of the switch of the first one and the second one ofthe second side switching elements after the transitioning of theassociated diode may be chosen in such a way that the diode charge iseffectively discharged and the reverse current slope may be conditionedeffectively via the switch in most operating conditions, while theprotection circuit ensures safe operation in extreme operatingconditions. A desired trade-off between the two protection means can bedesigned for, such that the provision of the protection circuit givesadditional design flexibility.

According to a further embodiment, the second side converter circuitcomprises two second side switching elements and the protection circuitcomprises a voltage source and two protection diodes coupled to thevoltage source, with each of the second side switching elements beingcoupled to one of the two protection diodes. With the protection diodesand the voltage source, a voltage level may be provided by theprotection circuit, to which the first one and the second one of thesecond side switching elements are coupled. When the voltage at one ofthe first one and the second one of the second side switching elementsexceeds said voltage level, the respective protection diode becomesconductive and the voltage is limited via a current through therespective protection diode to the voltage source.

In case the second side converter circuit has the two parallel branchesbetween the pair of second side DC terminals as described above, theprotection circuit may consist only of the voltage source and the twoprotection diodes. Accordingly, this way of protection may be achievedwith a very low number of circuit components, in particular with onevoltage source only for reference purposes. In case the second sideconverter circuit is an H bridge circuit or a transformer center tappedcircuit, as described above, a protection circuit may also beimplemented. However, one reference voltage source per second sideswitching element may be needed.

According to a further embodiment, the voltage source is coupled to oneof the pair of second side DC terminals or to one of the pair of firstside DC terminals. In particular, it may be coupled to the respectivelow potential terminal. By coupling the voltage source to one of thepair of second side DC terminals or to one of the pair of first side DCterminals, the power removed from the first one or the second one of thesecond side switching elements due to over-voltage protection can bere-fed into the DC/DC converter system and is not lost. In other words,the coupling of the voltage source allows for power regeneration bytransfer of clamped charges to the terminals the protection circuit iscoupled to. It can either be re-fed on the first side of the DC/DCconverter or on the second side of the DC/DC converter. The second sidemay be chosen as the sink of the re-fed power when the DC/DC converteris optimized for power transfer from the first side to the second side.The first side may be chosen as the sink of the re-fed power whenemphasis is put on particular power flow conditions from the second sideto the first side in a bi-directional DC/DC converter, such as thestarting of a DC generator coupled to the first side DC terminals usingpower from a battery coupled to the second side DC terminals. With thevoltage source being coupled to the one of the pair of second side DCterminals, the protection branches, respectively consisting of one ofthe two protection diodes and the voltage source, are in parallel withthe first one and the second one of the second side switching elementsof the second side converter circuit, respectively. In this way, thepower is split up between the respective parallel branches, preventingan over-voltage condition at the first one and the second one of thesecond side switching elements. It is possible to connect the one of thepair of second side DC terminals or the one of the pair of first side DCterminals, in particular the respective low potential terminal, toground. In this way, the voltage source is also connected to ground andprovides for a stable reference voltage.

It is also possible to couple the voltage source to the first side DCterminals or the second side DC terminals via a transformer forimpedance matching. Accordingly, the voltage level defining anover-voltage condition for the first one and the second one of thesecond side switching elements may be chosen to be different from theoperating voltage across the first or second side DC terminals.

The voltage source of the protection circuit may be controlled. In thisway, a constant voltage supply is ensured. Charge absorption by thevoltage source from the second side converter circuit is controlledtherewith, such that this charge does not increase the voltage value ofthe voltage source. Voltage control may be done via an additional DC/DCconverter. In case the voltage source is coupled to the pair of firstside DC terminals, galvanic insulation may be provided between thevoltage source and the pair of first side DC terminals.

Providing the protection circuit may eliminate the need to provide aconductive path from the high potential terminal to the low potentialterminal of the second side DC terminals at all times during the powertransfer from the second side DC terminals to the first side DCterminals where the second side converter circuit has current sourcecharacteristic. The current pushed by the current source characteristicmay be absorbed be the protection circuit. This may give moreflexibility in operating conditions when the first side convertercircuit is loaded from the second side and is not yet able to processlarge increments of transferred power before reaching a desiredoperating point.

According to a further embodiment, the at least one first side convertercircuit is a plurality of first side converter circuits connected inseries between the pair of first side DC terminals and wherein the atleast one second side converter circuit is a plurality of second sideconverter circuits connected in parallel between the pair of second sideDC terminals. In this way, the power transfer may be split up between aplurality of first side converter circuits and a plurality of secondside converter circuits, such that each of these converter circuits maybe designed for a lower power transfer capability than in the case ofonly one first side converter circuit and one second side convertercircuit. While the provisions of a plurality of converter circuitsincreases the number of components, reducing the power transfer capacityrequirements allows for using cheaper components, such that an overallcheaper design of the DC/DC converter may be achieved. Providing aplurality of converter circuits may extend the application fields of theDC/DC converter to higher power transfer levels, in particular to highervoltage levels.

It is also possible that the DC/DC converter comprises a plurality offirst side converter circuits and exactly one second side convertercircuit. It is also possible that the DC/DC converter comprises exactlyone first side converter circuit and a plurality of second sideconverter circuits.

According to a further embodiment, the DC/DC converter comprises aplurality of transformers, with each transformer coupling one of thefirst side converter circuits to one of the second side convertercircuits. In other words, the number of first side converter circuits isequal to the number of second side converter circuits. Each of the firstside converter circuits is coupled to exactly one second side convertercircuit via one transformer circuit. In this way, a plurality ofsub-modules is formed each consisting of one first side convertercircuit, one transformer and one second side converter circuit. Thesub-modules may be designed to account for a certain portion of themaximum power transfer capacity of the DC/DC converter. The number ofsub-modules may be 2, 4, 6 or 8, but is not limited thereto.

The plurality of sub-modules can be controlled in the same manner, i.e.with the same control signals. Accordingly, no additional controlefforts have to be undertaken in the case of providing multiplesub-modules. The parallel arrangement of the second side convertercircuits allows for a balancing effect between the second side convertercircuits in case of unequal loading of the sub-modules.

According to a further embodiment, in operation, a desired operatingvoltage across the pair of first side DC terminals may be at least 10times greater than an operating voltage across the pair of second sideDC terminals. The term desired operating voltage across the pair offirst side DC terminals is understood to be the operating voltage acrossthe pair of first side DC terminals that the DC/DC converter iscontrolled for. In other words, the desired operating voltage across thepair of first side DC terminals may be one control goal of the controlalgorithm of the DC/DC converter.

According to a further embodiment, in operation, a desired operatingvoltage across the pair of first side DC terminals is between 400V and800V, preferably between 500V and 700V.

According to a further embodiment, in operation, an operating voltageacross the pair of second side DC terminals is between 10V and 40V,preferably between 20V and 30V.

With these voltage levels, the DC/DC converter may be used forconverting the DC voltage supplied by a re-chargeable battery coupled tothe second side DC terminals for starting or supporting a DC generatorcoupled to the first side DC terminals. The re chargeable battery mayprovide DC voltage to other devices at a voltage level common invehicles. The DC/DC converter may be used bi-directionally. Therefore,it is possible to use the power generated by the DC generator for recharging the battery. The DC generator may be driven by a combustionmotor, such as a Diesel engine.

Above mentioned problem is further solved by the method of controlling agalvanically isolated DC/DC converter in accordance with claim 28.

The claimed method of controlling the galvanically isolated DC/DCconverter, which comprises at least one first side converter circuitcoupled between a pair of first side DC terminals and at least onesecond side converter circuit coupled between a pair of second side DCterminals, wherein the first side converter circuit has at least a firstand a second switching element, with each of the first and secondswitching elements comprising a switch and a diode connected inparallel, and wherein the second side converter circuit is controlled toalternate between two power transfer states for transferring power fromthe pair of second side DC terminals to the pair of first side DCterminals, the method comprising the steps of (a) operating the secondside converter circuit in one of the two power transfer states, with theone of the two power transfer states leading to the diode of aparticular one of the switching elements to be in a conductive state,(b) ending the one of the two power transfer states and putting thesecond side converter circuit in a state of no power transfer, (c)closing the switch of the particular one of the switching elements foran adaptation interval, and (d) putting the second side convertercircuit in the other one of the two power transfer states, with theother one of the two power transfer states leading to the diode of theother one of the switching elements to be in a conductive state. Themethod steps (a), (b), (c) and (d) are carried out in the order given.

The same advantages may be achieved with the claimed method ofcontrolling the galvanically isolated DC/DC converter as with thegalvanically isolated DC/DC converter as described above. It is pointedout that the embodiments and further features described above withrespect to the galvanically isolated DC/DC converter are equallyapplicable to the method of controlling a galvanically isolated DC/DCconverter in an analogous manner. It is again pointed out thatalternating between two power transfer states does not mean that one ofthese two power transfer states is present at any given point in time.Instead, a state of no power transfer is generally present between thetwo power transfer states when switching from one power transfer stateto the other.

According to a further embodiment, step (d) takes place a presetcommutation time after an end of the adaptation interval.

According to a further embodiment, an amount of power transfer iscontrolled by a duration of the two power transfer states. In otherwords, the amount of power transfer is controlled by a duration of astate of no power transfer between the two power transfer states whenswitching therebetween.

According to a further embodiment, the second side converter circuit hasat least two second side switching elements, with each of a first oneand a second one of the second side switching elements comprising aswitch and a diode connected in parallel, wherein the first sideconverter circuit is controlled to alternate between two forward powertransfer states for transferring power from the pair of first side DCterminals to the pair of second side DC terminals, the method comprisingthe steps of (k) putting the first side converter circuit in one of thetwo forward power transfer states, with the one of the two forward powerstates leading to a particular one of the diodes of the first one andthe second one of the second side switching elements to be in aconductive state, (l) closing the switch of the second side switchingelement which comprises the particular diode, (m) terminating the one ofthe two forward power transfer states, and (n) opening the closed switchafter the particular diode has transitioned from the conductive state toa blocking state. The method steps (k), (l), (m) and (n) are carried outin the order given.

The DC/DC converter as a whole may be controlled in such a way that thefirst side converter circuit is controlled to alternate between the twoforward power transfer states for forward power transfer or that thesecond side converter circuit is controlled to alternate between the twopower transfer states for reverse power transfer or that no powertransfer is present in the DC/DC converter. Accordingly, there may beoperating times of forward power transfer, operating times of reversepower transfer and operating times of no power transfer, yielding abi-directional DC/DC converter.

According to a further embodiment, the method further comprises betweensteps (m) and (n) the step of (m′) putting the first side convertercircuit in the other one of the two forward power transfer states, withstep (n) taking place a preset lag time after step (m′).

According to a further embodiment, step (l) takes place a preset delaytime after step (k). According to a further embodiment, the preset delaytime is greater than the preset lag time.

According to a further embodiment, an amount of power transfer iscontrolled by a duration of the two forward power transfer states.

It is explicitly pointed out that the nomenclature of a first side and asecond side, of a forward power transfer operation and a reverse powertransfer operation are not intended to be limiting in any way. They areused for distinguishing between the two operating directions of abi-directional DC/DC converter. However, depending on the particularimplementation of the invention and the particular circuit topology ofthe DC/DC converter, these power transfer directions may be calleddifferently. Only the interdependencies between the control of the DC/DCconverter and the circuit structure as claimed matters in this context.It is further explicitly pointed out that, within the scope of theinvention, a DC/DC converter may be a unidirectional DC/DC converter,wherein only the power transfer direction from the second side to thefirst side, in the nomenclature used herein, is implemented. Also, theDC/DC converter may be a bi-directional DC/DC converter, with only thepower transfer direction from the second side to the first side beingimplemented as described herein and the power transfer direction fromthe first side to the second side being implemented differently fromwhat is described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with regard to theexemplary embodiments shown in the accompanying figures, in which:

FIG. 1 shows a circuit diagram of a previous approach DC/DC converter.

FIG. 2 shows a circuit diagram of a DC/DC converter according to anexemplary embodiment of the invention.

FIG. 3 shows a timing diagram of the control signals applied to theDC/DC converter for a power transfer from the first side to the secondside according to an exemplary embodiment of the invention.

FIG. 4 shows a timing diagram of the control signals applied to theDC/DC converter for a power transfer from the second side to the firstside according to an exemplary embodiment of the invention.

FIG. 5 shows a circuit diagram of a DC/DC converter according to anotherexemplary embodiment of the invention.

FIG. 6 shows a circuit diagram of a DC/DC converter according to anotherexemplary embodiment of the invention.

FIG. 7 shows circuit diagrams of the transformer and the second sideconverter circuit of DC/DC converters according to further exemplaryembodiments of the invention.

FIG. 8 shows a timing diagram of the control signals applied to theDC/DC converter for a pre-charging operation from the second side to thefirst side according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a circuit diagram of a DC/DC converter according to anexemplary embodiment of the invention. The DC/DC converter 2 comprises apair of first side DC terminals 10 and a pair of second side DCterminals 60. The DC/DC converter 2 further comprises a first sideconverter circuit 20, a transformer 30 and a second side convertercircuit 40, which jointly couple the pair of first side DC terminals 10to the pair of second side DC terminals 60. In particular, the pair offirst side DC terminals 10 is coupled to the first side convertercircuit 20, which in turn is coupled to the transformer 30, which inturn is coupled to the second side converter circuit 40, which in turnis coupled to the pair of second side DC terminals 60.

The first side converter circuit 20 comprises an H bridge circuit havingfour insulated-gate bipolar transistors (IGBTs) and four diodes. Inparticular, the first IGBT 22 is coupled in parallel with the firstdiode 23, the second IGBT 24 is coupled in parallel with the seconddiode 25, the third IGBT 26 is coupled in parallel with the third diode27, and the fourth IGBT 28 is coupled in parallel with the fourth diode29. The first and second IGBTs 22, 24 are coupled in series between thepair of first side DC terminals 10, with the center point being coupledto one end of the first side transformer winding of the transformer 30.Equally, the third and fourth IGBTs 26, 28 are coupled in series betweenthe pair of first side DC terminals 10, with the center point beingcoupled to the other end of the first side transformer winding of thetransformer 30. In addition, a first side capacitor 21 is coupledbetween the pair of first side DC terminals 10. The respectivecombinations of IBGT and diode, i.e. IGBT 22/diode 23, IBGT 24/diode 25,IGBT 26/diode 27, and IGBT 28 I diode 29, are also referred to asswitching elements of the first side converter circuit or as first sideswitching elements 20 a, 20 b, 20 c, and 20 d.

The second side converter circuit 40 comprises two parallel branchesbetween the pair of second side DC terminals 60. A first branchcomprises a first inductive element 50 and a first MOSFET 41. A secondbranch comprises a second inductive element 52 and a second MOSFET 45.The connection point between the first inductance element 50 and thefirst MOSFET 41 is coupled to one end of the second side transformerwinding of the transformer 30. The connection point between the secondinductance element 52 and the second MOSFET 45 is coupled to the otherend of the second side transformer winding of the transformer 30. Thefirst and second MOSFETs 41, 45 are each depicted as consisting of aswitch, i.e. the channel of the MOSFET, and a parasitic diode, which isoriented anti-parallel to the respective switch. In particular, thefirst MOSFET 41 comprises the switch 42 and the diode 44. The secondMOSFET 45 comprises the switch 46 and the diode 48. Accordingly, theparallel circuit of switch and diode is a possible circuit symbolrepresentation of a MOSFET component. Also, a second side capacitor 54is coupled between the pair of second side DC terminals 60.

The operation of the DC/DC converter 2 for a power transfer from thepair of first side DC terminals 10 to the pair of second side DCterminal 60, i.e. a forward power transfer, is explained with referenceto the timing diagram of FIG. 3. FIG. 3 shows the relative timing of thecontrol signals applied to the first to fourth IGBTs 22, 24, 26 and 28as well as to the switches 42 and 46 of the first and second MOSFETs 41,45. It is also shown the time of the presence of the first forward powertransfer state PWM1/4 and the second forward power transfer statePWM2/3, which will be described below. A high state means that therespective IGBT or the switch of the respective MOSFET is controlled tobe conductive at the depicted point in time.

The timing diagram of FIG. 3 shows that the first and second IGBTs 22,24 are alternately put in a conductive state. In particular, each of thefirst and second IGBTs 22, 24 is in a conductive state for almost 50% ofthe time. In order to avoid a 15 short circuit condition between thepair of first side DC terminals 10, a down time T_(T) is established,wherein both the first and second IGBTs 22, 24 are open. Accordingly,the first and second IGBTs are never closed at the same point in time.The third and fourth IGBTs 26, 28 are controlled in a correspondingmanner. That is, each of the third and fourth IGBTs 26, 28 is conductivefor almost 50% of the time, with none of the third and fourth IGBTs 26,28 being conductive during the downtime T_(T) for avoiding short circuitconditions.

Whenever the first IGBT 22 and the fourth IGBT 28 are in a conductivestate, a current flow from the high potential terminal, depicted on top,of the pair of the DC terminals 10 through the first IGBT 22, throughthe first side transformer winding of the transformer 30, through thefourth IGBT 28 to the low potential terminal, depicted at the bottom, ofthe first side DC terminals 10 is established. This current flow flowsthrough the first side transformer winding of the transformer 30 in afirst direction, such that a first forward power transfer state PWM1/4is established. Correspondingly, whenever the second IGBT 24 and thethird IGBT 26 are in a conductive state, a second forward power transferstate PWM2/3 is established. In this case, the current flow flowsthrough the first side transformer winding of the transformer 30 in asecond direction. In particular, the current flow is established fromthe high potential terminal of the pair of first side DC terminals 10through the third IGBT 26, through the first side transformer winding ofthe transformer 30, through the second IGBT 24 to the low potentialterminal of the pair of the first side DC terminals 10.

The duration of the two forward power transfer states PWM1/4 and PWM2/3is set by the offset between the control signals for the first andsecond IGBTs 22, 24 and the control signals for the third and fourthIGBTs 26, 28, as can be seen graphically in FIG. 3. Accordingly, thisoffset or phase shift between these control signals is used forcontrolling the amount of power transferred from the pair of first sideDC terminals 10 to the pair of the second side DC terminals 60. In thisway, the voltage across the second side capacitor 54 can be keptconstant for a varying load coupled to the pair of second side DCterminals 60.

The current flow through the first side transformer winding of thetransformer 30 in the first forward power transfer state PWM1/4 inducesa current flow in the second side transformer winding of the transformer30, which leads to a current flow from the low potential terminal of thesecond side DC terminals 60 through the diode 44 of the first MOSFET 41,through the second side transformer winding of the transformer 30,through the second inductance element 52 to the high potential terminalof the pair of second side DC terminals 60. In other words, the firstforward power transfer state makes the diode 44 of the first MOSFET 41enter a conductive state, such that a current path between the pair ofsecond side DC terminal 60 through the second side transformer windingof the transformer 30 is established. The switch 42 of the first MOSFET41 is closed a preset delay time T_(V) after the first forward powertransfer state PWM1/4 is entered. From that moment on, current flowsthrough both the diode 44 and the switch 42 of the first MOSFET 41, suchthat the first MOSFET 41 as a whole imposes a lower voltage drop andlower losses, respectively, than the diode 44 alone.

When the first forward power transfer state PWM1/4 ends, the currentflow through the first side transformer winding of the transformer 30ends, which leads to the current flow through the second sidetransformer winding of the transformer 30 to end and to the diode 44 ofthe first MOSFET 41 to transition from the conductive state to ablocking state. Although the conduction through the diode 44 stops, theswitch 42 of the first MOSFET 41 is kept close. In this way, aconnection between the two sides of the diode 44 remains. In this way, acharge built up at the imperfect parasitic diode 44 will not result inan over-voltage condition when the diode 44 transitions to the blockingstate. In particular, the switch 42 of the first MOSFET 41 is controlledin a way to condition the current slope of the diode 44 for thetransitioning from the conductive state to the blocking state byproviding a parallel channel. The conditioning is achieved bycontrolling the MOSFET 41 in a way to exhibit an appropriate resistancealong its channel, i.e. along its switch 42. In this way, the negativeimpact of over-voltage spikes caused by the discharge of the built-upcharge at the diode 44 is eased. In particular, an abrupt ending of thereverse current caused by the built-up charge at the diode 44, which isprone to causing over-voltages at parasitic inductances, which areharmful to the electric components, is prevented.

After an interval of no power transfer, the second forward powertransfer state PWM2/3 is entered. The switch 42 of the first MOSFET 41is opened a preset lag time T_(N) after the second forward powertransfer state PWM2/3 is entered. The second forward power transferstate PWM2/3 makes the diode 48 of the second MOSFET 45 conductive,giving rise to a current flow from the low potential terminal, depictedat the bottom, of the pair of second side DC terminals 60 through thediode 48, through the second side transformer winding of the transformer30, through the first conductance element 50 to the high potentialterminal, depicted on top, of the pair of second side DC terminals 60.The switch 46 of the second MOSFET 45 is closed a preset delay timeT_(V) after the second forward power transfer state PWM2/3 is entered.With the preset delay time T_(V) being greater than the preset lag timeT_(N), it is ensured, that the switches 42 and 46 of the first andsecond MOSFET 41 and 45 are not closed at the same point in time duringthe forward power transfer. Therefore, no short circuit conditionbetween the pair of second side DC terminals 60 can arise.

From here on, the DC converter operates in the second forward powertransfer state in a manner analogous to the first forward power transferstate described above. Accordingly, the current through the secondMOSFET 45 is split up between the switch 46 and the diode 48 thereof,giving rise to a resistance voltage drop lower than that of the diode 48itself. When the second forward power transfer state PWM2/3 ends, thediode 48 transitions from the conductive state to a blocking state,while the switch 46 remains closed. In this way, the negative impact ofover-voltage spikes caused by the discharge of the built-up charge atthe diode 44 is eased. In particular, an abrupt ending of the reversecurrent caused by the built-up charge at the diode 44, which is prone tocausing over-voltages at parasitic inductances, which are harmful to theelectric components, is prevented by providing the parallel channelthrough the switch 46. After an interval of no power transfer, the firstforward power transfer state PWM1/4 is entered again. The switch 46 ofthe second MOSFET 45 is opened the preset lag time T_(N) after the firstforward power transfer state PWM1/4 is entered. And the switch 42 of thefirst MOSFET 41 is closed a preset delay time T_(V) after the firstforward power transfer state PWM1/4 is entered.

This operation continues with the DC/DC converter 2 alternating betweenthe two power transfer states PWM1/4 and PWM2/3, as long as powertransfer from the first side DC terminals 10 to the second side DCterminals 60 is desired.

The operation of the DC/DC converter 2 of FIG. 2 is described for thepower transfer from the pair of second side DC terminals 60 to the pairof first side DC terminals 10 with reference to FIG. 4. FIG. 4 shows atiming diagram of the control signals applied to the first and secondMOSFETs 41 and 45 as well as the first to fourth IGBTs 22, 24, 26 and28. A high state of the respective signals indicates that the respectiveIGBT or the respective switch 42, 46, i.e. the respective channel, ofthe MOSFET is controlled to be in a conductive state.

As the case of power being transferred from the second side DC terminals60 to the first side DC terminals 10 is described, which is alsoreferred to as a reverse power transfer, reference is first made to thesecond side converter circuit 40 as the source of power for thetransformer 30. FIG. 4 shows that both the switch 42 of the first MOSFET41 and the switch 46 of the second MOSFET 45 alternate between an openand a closed state. It is also shown that both of these switches areclosed for more than 50% of the total time. The control signals for theswitches 42 and 46 are phase shifted with respect to each other, suchthat both switches 42 and 46 are never open at the same time. However,the closed state of the switches 42 and 46 overlap. In this way, it isensured that there always is a conductive path from the high potentialterminal of the second side DC terminals 60 to the low potentialterminal of the second side DC terminals 60. In this way, the currentflow imposed by the first and second inductive elements 50 and 52 canfind its way through the second side converter circuit 40 and does notgive rise to dangerous voltage peaks in the second side convertercircuit 40.

When both the switch 42 and the switch 46 are in a closed state, nocurrent flows through the second side transformer winding of thetransformer 30. Accordingly, a state of no power transfer isestablished. When one of the two switches 42 and 46 is closed, while theother switch is open, a current flow through the second side transformerwinding of the transformer 30 is established and power transfer to thefirst side takes place. Depending which one of the switches 42 and 46 isclosed and which one is open, the current flows through the second sidetransformer winding of the transformer 30 in a respective direction,giving rise to one of two power transfer states. As these power transferstates are associated with the power transfer from the pair of secondside DC terminals 60 to the pair of first side DC terminals 10, they arealso referred to as reverse power transfer states.

In particular, when the switch 42 of the first MOSFET 41 is closed andthe switch 46 of the second MOSFET 45 is open, a current flow from thehigh potential terminal of the second side DC terminals 60, depicted ontop, through the second inductance element 52, through the second sidetransformer winding of the transformer 30 and through the switch 42 tothe low potential terminal, depicted at the bottom, of the second sideDC terminals 60 is established. This scenario is referred to as thefirst reverse power transfer state. It gives rise to a current flow inthe first side transformer winding of the transformer 30 and the firstside converter circuit 20. In particular, a current flow from the lowpotential terminal of the first side DC terminals 10, depicted at thebottom, through the fourth diode 29, through the first side transformerwinding of the transformer 30 and through the first diode 23 to the highpotential terminal of the first side DC terminals 10 is established. Inthis way, power is transferred to the pair of first side DC terminals inthe first reverse power transfer state.

As can be seen from FIG. 4, shortly before the first reverse powertransfer state is entered, a control pulse is applied to the second andthird IGBTs 24, 26. In other words, the IGBTs 24 and 26 are closed for aduration T_(P), referred to as an adaptation interval T_(P), with thisadaptation interval T_(P) ending a commutation time T_(K) before thefirst reverse power transfer state is entered. In this way, a currentflow through the first side transformer winding of a transformer 30 isgenerated in the same direction as the current flow that will be presentduring to first reverse power transfer state. Accordingly, a currentflow in the expected direction is already present in the first sidetransformer winding of the transformer 30 when the first reverse powerstate is entered.

As can also be seen from FIG. 4, the conductive state of the second andthird IGBTs 24, 26 ends a commutation time T_(K) before the start of thefirst reverse power transfer state. The commutation time T_(K) is apreset interval. This preset commutation time T_(K) allows for thecurrent flow to commutate from its path through the second and thirdIGBTs 24, 26 to the first and fourth diodes 23, 29. In this way, thefirst and fourth diodes are put in a conductive state before the onsetof the first reverse power transfer state, such that the power transferexperiences a smooth beginning. In particular, the parasiticcapacitances of the first and fourth diodes 23, 29 are charged beforethe beginning of the first reverse power transfer state, such that afavorable impedance is already present for the current induced from thesecond side converter circuit.

The second and third IGBTs 24, 26 are in a closed state for theadaptation interval T_(P). The adaptation interval is chosen in such away that the current flow generated during this interval helps the powertransfer in an optimized manner. For this purpose, it is taken intoaccount that the current flow increases in magnitude during theadaptation interval T_(P), whereas it decreases during the presetcommutation time T_(K). In the particular embodiment of FIGS. 2 and 4,the adaptation interval T_(P) is chosen in such a way that undesiredvoltage peaks within the first side converter circuit 20 are kept at ajust acceptable limit at the onset of the reverse power transfer statefor the case of maximum power transfer from the pair of second side DCterminals 60 to the pair of first side DC terminals 10.

The operation of the DC/DC converter 2 in the second reverse powertransfer state is analogous to their first reverse power transfer statedescribed above. While both of the switch 42 of the first MOSFET 41 andthe switch 46 of the second MOSFET 45 are closed, first and fourth IGBTs22 and 28 are put in a conductive state for the adaptation intervalT_(P). This gives rise to a current flow from the high potentialterminal of the first side DC terminals 10 through the first IGBT 22,through the first side transformer winding of the transformer 30 andthrough the fourth IGBT 28 to the low potential terminal of the firstside DC terminals 10. The switch 42 of the first MOSFET 41 is opened thepreset commutation time T_(K) after the first and fourth IGBTs 22 and 28are opened. As a consequence a current flow from the high potentialterminal of the second side DC terminals 60 through the first inductanceelement 50, through the second side transformer winding of thetransformer 30 and through the switch 46 of the second MOSFET 45 to thelow potential terminal of the first side DC terminals 60 is established.This current flow gives rise to a current flow in the first sidetransformer winding of the transformer 30 in the same direction aspreviously established during the adaptation interval T_(P), which inturn gives rise to a current flow from the low potential terminal of thefirst side DC terminals 10 through the second diode 25, through thefirst side transformer winding of the transformer 30 and through thethird diode 27 to the high potential terminal of the first side DCterminals 10. In this way, a power transfer from the pair of second sideDC terminals 60 to the pair of first side DC terminals 10 isestablished. With the conditioning of the first side converter circuit20, this power transfer has a smooth onset, as described above for thefirst reverse power transfer state.

Another exemplary embodiment of a DC/DC converter 2 in accordance withthe invention is described with reference to FIG. 5. The DC/DC converter2 of FIG. 5 comprises two power transfer sub-modules. The first powertransfer sub-module comprises a first side converter circuit 20, atransformer 30 and a second side converter circuit 40. These componentsare identical to the respective components of FIG. 2 and are thereforedesignated with the same reference numerals. The second power transfersub-module comprises a further first side converter circuit 120, afurther transformer 130 and a further second side converter circuit 140.These components are also identical to the first side converter circuit20, the transformer 30 and the second side converter circuit 40 of FIG.2. Their reference numerals correspond to the reference numerals of therespective components of the DC/DC converter 2 of FIG. 2, with eachreference numeral being increased by 100. For brevity, an in-depthdiscussion of the structure is omitted.

Attention is now drawn to how the first side converter circuits 20, 120are coupled to the pair of first side DC terminals 10 and to how thesecond side converter circuits 40 and 140 are coupled to the second sideDC terminals 60. In particular, the first side converter circuit 20 andthe first side converter circuit 120 are coupled in series between thepair of first side DC terminals 10. In parallel with the seriesconnection of the first side converter circuits 20 and 120, anadditional first side capacitor 12 is coupled between the pair of firstside DC terminals 10. The second side converter circuit 40 and thesecond side converter circuit 140 are coupled in parallel between thepair of second side DC terminals 60. Also, an additional second sidecapacitor 62 is coupled in parallel with the second side convertercircuits 40 and 140 between the second side DC terminals 60. As isapparent from basic circuit theory, the second side capacitor 54 of thesecond side converter circuit 40, the second side capacitor 154 of thesecond side converter circuit 140 and the additional second sidecapacitor 62 are arranged in parallel and may therefore be replaced byone single second side capacitor. Also, the additional first sidecapacitor 12 can be dispensed with if the capacitance values of each ofthe first side capacitors 21 and 121 is increased by twice thecapacitance value of the additional first side capacitor 12, when anequal electric behavior is desired.

The DC/DC converter 2 of FIG. 5 is controlled via generally the samecontrol signals as the DC/DC converter 2 of FIG. 2. Accordingly, thecontrol signals depicted in FIGS. 3 and 4 can be used withoutmodification for the control of the DC/DC converter 2 of FIG. 5. In casethe voltage across the second side DC terminals 60 is controlled, bothsecond side converter circuits 40, 140 may be fed with the same controlpattern. In case the voltage across the first side DC terminals 10 iscontrolled, each sub-module is controlled with its own voltage controlloop, which may result in different control patterns for eachsub-module, such that any kind of asymmetric behavior between thesub-modules may be compensated for by those individual control patterns.

However, with the first power transfer sub-module and the second powertransfer sub-module being identical with regard to their circuitstructure, the power amount transfer per sub-module is half of the poweramount transferred via the respective components of the DC/DC converter2 of FIG. 2, assuming an overall equal amount of power transfer.Accordingly, the individual circuits and components have to bedimensioned for half the power transfer capacity. In particular, on thefirst side, the voltage drop across each one of the first side convertercircuits 20 and 120 is halved as compared to the DC/DC converter 2 ofFIG. 2, assuming the same total voltage drop across the pair of firstside DC terminals 10. Seen from a different angle, greater voltage dropsacross the first side DC terminals 10 can be supported for a givenvoltage drop capacity of a first side converter circuit.

With the series connection of the first side converter circuits 20 and120 and the parallel connection of the second side converter circuits 40and 140, a large voltage difference between the pair of first side DCterminals 10 and the pair of second side DC terminals 60 can be achievedin a very elegant manner.

Another exemplary embodiment of the invention is described with regardto FIG. 6. FIG. 6 depicts the right side, i.e. the second side, of theDC/DC converter 2 of FIG. 5 extended by a protection circuit 70. Theprotection circuit 70 comprises four protection diodes 72, 74, 172 and174 and a voltage source 80. The first and second protection diodes 72and 74 are associated with the second side converter circuit 40. Thethird and fourth protection diodes 172 and 174 are associated with thesecond side converter circuit 140.

The first protection diode 72 is coupled between the connection point ofthe first MOSFET 41 and the first inductance element 50 of the firstside converter circuit 40 and the voltage source 80. The secondprotection diode 74 is coupled between the connection point of thesecond MOSFET 45 and the second inductance element 52 of the second sideconverter circuit 40 and the voltage source 80.

The third protection diode 172 is coupled between the connection pointof the first MOSFET 141 and the first inductance element 150 of thefirst side converter circuit 140 and the voltage source 80. The fourthprotection diode 17 4 is coupled between the connection point of thesecond MOSFET 145 and the second inductance element 152 of the secondside converter circuit 140 and the voltage source 80.

Accordingly, each MOSFET of the second side converter circuits 40 and140 is coupled to the same one of the terminals of the voltage source 80of the protection circuit 70 via a respective protection diode. Theother terminal of the voltage source 80, i.e. the terminal of thevoltage source 80 that is not coupled to the protection diodes, iscoupled to the low potential terminal of the pair of second side DCterminals 60. Being connected to the low potential terminal of the pairof second side DC terminals 60, the other terminal of the voltage source80 is also coupled to the connection point between the first and secondMOSFETs 41 and 45 of the second side converter circuit 40 as well as tothe connection point of the first and second MOSFETs 141 and 145 of thesecond side converter circuit 140. In this way, if the voltage acrossone of the MOSFETs exceeds the voltage of the voltage source 80 plus theforward voltage drop of a protection diode, the protection diode becomesconductive and the voltage across the respective MOSFET is limited tothis threshold level via a current through the respective protectiondiode. In this way, an additional layer of over-voltage protection isprovided for the MOSFETs of the second side converter circuits 40 and140. The voltage source 80 sinks the charge removed from the MOSFETexperiencing an over-voltage condition. In the exemplary embodiment ofFIG. 6, the voltage source 80 then sinks the charge to the additionalsecond side capacitor 62, such that the power is not removed from thesystem and dissipated, but fed back into the second side convertercircuits.

The voltage value of the voltage source 80 is chosen in such a way thatit is greater than or equal to twice the maximum desired operatingvoltage across the pair of second side DC terminals 60 for theparticular DC/DC converter embodiment. In this way, it is ensured thatno energy is extracted from the second side converter circuit 40 by theprotection circuit 70 in a case where the voltage across the second sideDC terminals 60 is still in the desired range for the momentaryoperating conditions.

It is explicitly pointed out that the protection circuit 70 may equallybe applied to the DC/DC converter 2 of FIG. 2 having only one secondside converter circuit. In this case, only the first and secondprotection diodes 72 and 74 as well as the voltage source 80 would bepresent forming the protection circuit 70. Also, the protection circuit70 may be present in embodiments having more than two second sideconverter circuits. In this case, two protection diodes per second sideconverter circuit are present.

With regard to FIG. 8, it is further described a method of pre-chargingthe first side capacitances via the second side converter circuits 40and 140 of FIG. 6. Again, this method may equally be carried out in aDC/DC converter having only one second side converter circuit 40. Such apre-charging may be carried out in order to establish a voltage levelacross the first side capacitances that ensures that the energytransferred from the pair of second side DC terminals 60 to the pair offirst side DC terminals 10 may be properly absorbed on the first side.It may therefore be performed, before the stationary energy transferfrom the pair of second side DC terminals 60 to the pair of first sideDC terminals 10, as described with regard to the timing diagram of FIG.4, is carried out.

For this pre-charging, only the switch 42 of the first MOSFET 41 and theswitch 46 of the second MOSFET 45 are operated, as shown in FIG. 8. Theswitch 46 is closed for a charging interval T_(S). During this interval,a current flow from the high potential terminal of the second side DCterminals 60 through the second inductance element 52 and the switch 46to the low potential terminal of the second side DC terminals isestablished. After the charging interval T_(S), when the switch 46 isopened, the current slowly starts to commutate from the switch 46 to thesecond side transformer winding of the transformer 30 and the firstinductance element 50. In this way, only a small amount of energy istransferred to the first side converter circuit, which can be absorbedthere for pre-charging the first side capacitances. Since thecommutation of the current is a gradual process, a voltage peak arisesat the first MOSFET 45, which is limited to an acceptable value via theprotection circuit 70, in particular via the second protection diode 74.In this way, the second side converter circuit 40 is protected againstharmful voltage peaks during pre-charging the first side of the DC/DCconverter.

The pre-charging process continues in an analogous manner, alternatingbetween closing the switch 42 of the first MOSFET 41 and the switch 46of the second MOSFET 45 for respective pre-charging intervals T_(S). Inbetween these pre-charging intervals, none of the switches 42 and 46 isclosed, as depicted in FIG. 8. The pre-charging process is carried outuntil the voltage across the first side DC terminals 10 reaches adesired operating point. The energy absorbed by the protection circuit70 is not lost, but re-fed back into the system in the topology of FIG.6. It is also possible to connect the power source 80 to the lowpotential terminal of the pair of first side DC terminals 10, such thatthe energy absorbed by the protection circuit 70 helps directly incharging the first side of the DC/DC converter, thereby achieving afaster pre-charging of the first side.

The pre-charging interval T_(S) may be chosen in a way to ensure thatthe protection circuit 70 is not over-loaded. Accordingly, the durationof the pre-charging depends on the values of the circuit components ofthe second side converter circuit 40, the transformer 30 as well as thedimensions of the protection circuit 30.

FIG. 7 shows two alternatives for second side converter circuits thatcan be substituted for each of the second side converter circuits 40 and140 shown in FIGS. 2 and 5. FIG. 7 A shows a second side convertercircuit 340, which comprises an H bridge circuit. An inductance element350 and the H bridge circuit are coupled in series between the pair ofsecond side DC terminals 60. A second side capacitor 354 is coupled inparallel with said series connection between the pair of second side DCterminals 60. The H bridge circuit comprises a first MOSFET 341, asecond MOSFET 345, a third MOSFET 375 and a fourth MOSFET 371.

The second side converter circuit 340 can be controlled by a samecontrol signal as the second side converter circuits 40 and 140. Inparticular, the control signal supplied to the first MOSFET 41 of thesecond side converter circuit 40, which controls the switch 42 thereof,is supplied to the first MOSFET 341 and the fourth MOSFET 371.Correspondingly, the control signal supplied to the second MOSFET 45 ofthe second side converter circuit 40, which controls the switch 46thereof, is supplied to the second MOSFET 345 and the third MOSFET 375of the second side converter circuit 340.

It is pointed out that the third and fourth MOSFETs 375 and 371 may bereplaced with capacitors.

The circuit structure of the second side converter circuit 340corresponds to the circuit structure of the second side convertercircuit 240 of FIG. 1. Accordingly, the second side converter circuit240 of FIG. 1 may also be controlled in accordance with the inventionand may therefore be part of an inventive DC/DC converter.

FIG. 7B shows another embodiment of a second side converter circuit 440,which is referred to as a transformer center tapped circuit. The secondside converter circuit 440 works with another implementation of atransformer 430, which provides the center point of the second sidetransformer winding as a connection point for the second side convertercircuit 440. This center point is coupled to the low potential terminalof the second side DC terminals 60 via an inductance element 450. Thetwo ends of the second side transformer winding of the transformer 430are coupled to the high potential terminal of the second side DCterminals 60 via the first MOSFET 441 and the second MOSFET 445 of thesecond side converter circuit 440, respectively. The second sideconverter circuit 440 further comprises a second side capacitor 454coupled between the pair of second side DC terminals 60. It is alsopossible to flip the direction of the first and second MOSFETs 441 and445 to achieve a reversed polarity at the pair of second side DCterminals 60.

Again, the second side converter circuit 440 can be controlled with thesame control signals as the second side converter circuits 40 and 140 ofFIGS. 2 and 5 if the transformer ratio of the transformer 30 equals thetransformer ratio of the transformer 430 when looking at the totalnumber of second side transformer windings thereof. In particular, thecontrol signal for the first MOSFET 41 of the second side convertercircuit 40 may equally be applied to the first MOSFET 441 of the secondside converter circuit 440. Also, the control signal for the secondMOSFET 45 of the second side converter circuit 40 may equally be appliedto the second MOSFET 445 of the second side converter circuit 440. Ifdiffering transformer ratios are present, the control signals areadjusted accordingly.

It is further pointed out that, in an alternative embodiment, not shownthroughout the figures, the first side converter circuit 20 may have anadjusted circuit topology. In particular, the third IGBT 26 and thethird diode 27 may be replaced by a capacitor. Equally, the fourth IGBT28 and the fourth diode 29 may be replaced by a capacitor. Accordingly,only the first and second IGBTs 22 and 24 would have to be supplied withthe discussed control signals.

It is explicitly pointed out that the nomenclature of a first side and asecond side, of a forward power transfer operation and a reverse powertransfer operation are not intended to be limiting in any way for theexemplary embodiments described. They are used for distinguishingbetween the two operating directions of a bi-directional DC/DCconverter. However, depending on the particular implementation of theinvention and the particular circuit topology of the DC/DC converter,these power transfer directions may be called differently. Only theinterdependencies between the control of the DC/DC converter and thecircuit structure as described matters in this context. It is furtherexplicitly pointed out that, within the scope of the invention,exemplary DC/DC converters are herewith also disclosed that areuni-directional DC/DC converters, wherein only one of the two powertransfer directions is implemented. In other words, it is not anecessary feature of the embodiments described that both the control ofFIG. 3 for one power transfer direction and the control of FIG. 4 forthe other power transfer direction are implemented. In case only one ofthe power transfer directions is implemented, some of the circuitelements may be eliminated from the DC/DC converter. As an example,should only the power transfer from the pair of first side DC terminals10 to the pair of second side DC terminals 60 be implemented and noreverse power transfer be required for the particular application, thefirst to fourth diodes 23, 25, 27 and 29 could be dispensed with.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationto the teachings of the invention without departing from the essentialscope thereof. Therefore, it is intended that the invention not belimited to the particular embodiments disclosed, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A galvanically isolated DC/DC converter (2), comprising: at least onefirst side converter circuit (20) coupled between a pair of first sideDC terminals (10), the first side converter circuit (20) having at leasta first and a second switching element (20 a, 20 b), with each of thefirst and second switching elements (20 a, 20 b) comprising a switch(22, 24) and a diode (23, 25) connected in parallel, and at least onesecond side converter circuit (40) coupled between a pair of second sideDC terminals (60), wherein, when the DC/DC converter (2) is in powertransfer operation from the pair of second side DC terminals (60) to thepair of first side DC terminals (10), the second side converter circuit(40) is adapted to alternate between two power transfer states, whereina conductive state of the diode of one of the first and second switchingelements (20 a, 20 b) is the result of one of the two power transferstates, with the first side converter circuit (20) being controlled suchthat the switch of the respectively other of the first and secondswitching elements (20 a, 20 b) is closed for an adaptation interval(T_(P)) prior to a beginning of the one of the two power transferstates.
 2. The galvanically isolated DC/DC converter (2) according toclaim 1, wherein the first side converter circuit (20) is an H bridgecircuit, which comprises the first and second switching elements (20 a,20 b) and further comprises a third and a fourth switching element (20c, 20 d), with each of the third and fourth switching elements (20 c, 20d) comprising a switch (26, 28) and a diode (27, 29) in parallel,wherein a conductive state of the diodes of two of the first to fourthswitching elements (20 a, 20 b, 20 c, 20 d) is the result of one of thetwo power transfer states, with the first side converter circuit (20)being controlled such that the switches of the respectively other two ofthe first to fourth switching elements (20 a, 20 b, 20 c, 20 d) areclosed for the adaptation interval (T_(P)) prior to the beginning of theone of the two power transfer states.
 3. The galvanically isolated DC/DCconverter (2) according to claim 1, wherein the second side convertercircuit (40) is adapted to be in a state of no power transfer betweenthe two power transfer states, with the adaptation interval (T_(P))being during the state of no power transfer.
 4. The galvanicallyisolated DC/DC converter (2) according to claim 1, wherein the firstside converter circuit (20) is controlled such that the adaptationinterval (T_(P)) ends a preset commutation time (T_(K)) before thebeginning of the one of the two power transfer states.
 5. Thegalvanically isolated DC/DC converter (2) according to claim 4, whereinthe preset commutation time (T_(K)) is chosen such that the conductivestate of the diode of the one of the first and second switching elements(20 a, 20 b) is present at the beginning of the one of the two powertransfer states.
 6. The galvanically isolated DC/DC converter (2)according to claim 1, wherein the adaptation interval (T_(P)) has apreset duration.
 7. The galvanically isolated DC/DC converter (2)according to claim 1, wherein a duration of the adaptation interval(T_(P)) is dependent on the operating point of the DC/DC converter. 8.The galvanically isolated DC/DC converter (2) according to claim 1,wherein each of the switches of the first side converter circuit (20) isan insulated-gate bipolar transistor.
 9. The galvanically isolated DC/DCconverter (2) according to claim 1, wherein the first side convertercircuit (20) has voltage source characteristics.
 10. The galvanicallyisolated DC/DC converter (2) according to claim 1, wherein the secondside converter circuit (40) has current source characteristics.
 11. Thegalvanically isolated DC/DC converter (2) according to claim 1, whereinthe second side converter circuit (40) has two parallel branches betweenthe pair of second side DC terminals (60), each branch comprising aninductance element (50, 52) and a second side switching element (41,45).
 12. The galvanically isolated DC/DC converter (2) according toclaim 1, wherein the second side converter circuit (40) is an H bridgecircuit or a transformer center tapped circuit having at least twosecond side switching elements.
 13. The galvanically isolated DC/DCconverter (2) according to claim 11, wherein each of the second sideswitching elements is a MOSFET.
 14. The galvanically isolated DC/DCconverter (2) according to claim 1, wherein the second side convertercircuit has at least two second side switching elements (41, 45), witheach of a first one and a second one of the second side switchingelements (41, 45) comprising a switch (42, 46) and a diode (44, 48)connected in parallel, wherein, when the DC/DC converter (2) is inforward power transfer operation from the pair of first side DCterminals (10) to the pair of second side DC terminals (60), the diodes(44, 48) of the first one and the second one of the second sideswitching elements (41, 45) are alternately in a conductive state, witheach of the first one and the second one of the second side switchingelements being controlled such that a closed state of the respectiveswitch extends beyond a transitioning of the diode of the same secondside switching element from the conductive state to a blocking state.15. The galvanically isolated DC/DC converter (2) according to claim 14,wherein the respective switch of each of the first one and the secondone of the second side switching elements (41, 45) is controlled tocondition a slope of a discharge current of the diode of the same secondside switching element during the transitioning thereof from theconductive state to the blocking state.
 16. The galvanically isolatedDC/DC converter (2) according to claim 14, wherein each of the first oneand the second one of the second side switching elements (41, 45) iscontrolled such that the respective switch is in the conductive stateduring substantially the whole time the diode of the same second sideswitching element is in the conductive state.
 17. The galvanicallyisolated DC/DC converter (2) according to claim 14, wherein the firstside converter circuit (20) is adapted to alternate between two forwardpower transfer states (PWM1/4, PWM2/3), wherein each forward powertransfer state leads to the conductive state of one of the diodes (44,48) of the first one and the second one of the second side switchingelements (41, 45), with each of the first one and the second one of thesecond side switching elements (41, 45) being controlled such that therespective switch is opened a preset lag time (T_(N)) after therespective one of the two forward power transfer states is entered thatleads to the conductive state of the diode of the other second sideswitching element.
 18. The galvanically isolated DC/DC converter (2)according to claim 14, wherein the first side converter circuit (20) isadapted to alternate between two forward power transfer states (PWM1/4,PWM2/3), wherein each forward power transfer state leads to theconductive state of one of the diodes (44, 48) of the first one and thesecond one of the second side switching elements (41, 45), with each ofthe first one and the second one of the second side switching elements(41, 45) being controlled such that the respective switch is closed apreset delay time (T_(V)) after the respective one of the two forwardpower transfer states is entered that leads to the conductive state ofthe diode of the same second side switching element.
 19. Thegalvanically isolated DC/DC converter (2) according to claim 18, whereinthe preset delay time (T_(V)) is greater than the preset lag time(T_(N)).
 20. The galvanically isolated DC/DC converter (2) according toclaim 1, wherein the second side converter circuit (40) is coupled to aprotection circuit (70).
 21. The galvanically isolated DC/DC converter(2) according to claim 20, wherein the second side converter circuit(40) comprises two second side switching elements (41, 45) and theprotection circuit comprises a voltage source (80) and two protectiondiodes (72, 74) coupled to the voltage source (80), with each of thesecond side switching elements (41, 45) being coupled to one of the twoprotection diodes (72, 74).
 22. The galvanically isolated DC/DCconverter (2) according to claim 21, wherein the voltage source (80) iscoupled to one of the pair of second side DC terminals (60) or to one ofthe pair of first side DC terminals (10).
 23. The galvanically isolatedDC/DC converter (2) according to claim 1, wherein the at least one firstside converter circuit is a plurality of first side converter circuits(20, 120) connected in series between the pair of first side DCterminals (10) and wherein the at least one second side convertercircuit is a plurality of second side converter circuits (40, 140)connected in parallel between the pair of second side DC terminals (60).24. The galvanically isolated DC/DC converter (2) according to claim 23comprising a plurality of transformers (30, 130), with each transformercoupling one of the first side converter circuits to one of the secondside converter circuits.
 25. The galvanically isolated DC/DC converter(2) according to claim 1, wherein, in operation, a desired operatingvoltage across the pair of first side DC terminals (10) is at least 10times greater than an operating voltage across the pair of second sideDC terminals (60).
 26. The galvanically isolated DC/DC converter (2)according to claim 1, wherein, in operation, a desired operating voltageacross the pair of first side DC terminals (60) is between 400V and800V, preferably between 500V and 700V.
 27. The galvanically isolatedDC/DC converter (2) according to claim 1, wherein, in operation, anoperating voltage across the pair of second side DC terminals (10) isbetween 10V and 40V, preferably between 20V and 30V.
 28. A method ofcontrolling a galvanically isolated DC/DC converter (2), which comprisesat least one first side converter circuit (20) coupled between a pair offirst side DC terminals (10) and at least one second side convertercircuit (40) coupled between a pair of second side DC terminals (60),wherein the first side converter circuit (20) has at least a first and asecond switching element (20 a, 20 b), with each of the first and secondswitching elements (20 a, 20 b) comprising a switch (22, 24) and a diode(23, 25) connected in parallel, and wherein the second side convertercircuit (40) is controlled to alternate between two power transferstates for transferring power from the pair of second side DC terminals(60) to the pair of first side DC terminals (10), the method comprisingthe steps of: (a) operating the second side converter circuit in one ofthe two power transfer states, with the one of the two power transferstates leading to the diode of a particular one of the switchingelements to be in a conductive state, (b) ending the one of the twopower transfer states and putting the second side converter circuit in astate of no power transfer, (c) closing the switch of the particular oneof the switching elements for an adaptation interval (T_(P)), and (d)putting the second side converter circuit in the other one of the twopower transfer states, with the other one of the two power transferstates leading to the diode of the other one of the switching elementsto be in a conductive state.
 29. The method according to claim 28,wherein step (d) takes place a preset commutation time (T_(K)) after anend of the adaptation interval (T_(P)).
 30. The method according toclaim 28, wherein an amount of power transfer is controlled by aduration of the two power transfer states.